2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD; dioxin), an extremely potent rodent carcinogen has also been shown to exhibit a wide spectrum of additional biological effects in a variety of species. These effects include alterations in metabolic pathways, developmental and reproductive changes, and immunotoxicity (Whitlock, J. (1994) Trends Endocrinol. Metab. 5:183-188; Hanson, C. and Smilalowicz, R. (1994) Toxicology 88:213-224). Despite extensive research, the molecular basis of dioxin toxicity remains poorly understood. Experimental evidence suggests that dioxin triggers its variety of biological responses by altering the expression of specific TCDD-responsive genes. This sequence of events is initiated when dioxin binds to the aromatic hydrocarbon receptor (Ah) and is followed by the interaction of the activated ligand-receptor complex with specific DNA response elements in the target genes. Among the TCDD-responsive genes are members of the P450 family, metabolic pathway enzymes, growth factor receptors including epidermal growth factor receptor, and several proteins associated with proinflammatory states such as interleukin 1B and plasminogen activator inhibitor 2 (Asman, D. et al. (1993) J. Biol. Chem. 268:12530-12536; Nebert, D. et al. (1991) Pharmacogenesis 1:68-78; and Sutter, T. et al. (1991) Science 254:415-418). Although it is still highly speculative whether these genes are actively involved in carcinogenesis, it appears that dioxin may alter important cell cycle checkpoints in a variety of regulatory systems. Interestingly, estrogen treatment blocks the TCDD-induced accumulation of several of these gene products in MCF-7 breast cancer cells (Kharat, I. and Saatcioglu, F. (1996) J. Biol. Chem. 271:10533-10537). Since TCDD has been shown to decrease estrogen-inducible gene products, it appears that two way cross-talk occurs between the intracellular signaling pathways involving steroids and aromatic hydrocarbons.
Using a PCR-based technique to identify differentially expressed genes, a novel cDNA called 25-Dx was isolated from rat livers exposed to TCDD (Selmin, O. et al. (1996) Carcinogenesis 17:2609-2615). Expression of 25-Dx in liver was enhanced in a dose dependent fashion over a broad range of TCDD exposures. Northern analysis of untreated rat tissues demonstrated additional 25-Dx expression in brain, lung, kidney and normal liver. Expression of 25-Dx was low in testis and spleen and undetectable in heart and skeletal muscle.
The protein sequence of 25-Dx suggested its identification as a new member of the cytokine/growth factor/prolactin receptor superfamily (Selmin et al., supra). The 25-Dx sequence contains a hydrophobic region which is 71% homologous to the transmembrane domain of the consensus IL-6 receptor sequence. In addition, an eight amino acid proline-rich region at amino acids 62 to 70 in the putative cytoplasmic domain of the 25-Dx protein is conserved within the receptor superfamily. This proline-rich region is necessary for the association of tyrosine kinases with members of the cytokine/growth hormone/prolactin receptor superfamily. The 25-Dx sequence contains several tyrosine residues which provide potential phosphate acceptor sites during receptor activation. 25-Dx may thus be a mediator of a signal transduction pathway (Selmin et. al, supra). Furthermore, as TCDD exposure modulates immune and inflammatory responses, Selmin et al. (supra) propose that 25-Dx may play a role in TCDD immunotoxicity.
A cDNA sequence encoding a protein with a 76% amino acid homology to 25-Dx has been identified in porcine liver microsomes and vascular smooth muscle cells (Falkenstein, E. et al. (1996) Biochem. Biophys. Res. Commun. 229:86-89). The protein sequence encoded by this cDNA matches the N-terminal and internal CNBr fragments of a partially sequenced progesterone membrane binding protein isolated from porcine liver membranes (Meyer, C. et al. (1996) Eur. J. Biochem. 239:726-731). Expression of this putative steroid membrane binding protein is tissue-specific and is similar to that observed for 25-Dx: high in brain, liver, lung and kidney, low in heart and spleen. Falkenstein et al. point out that this protein appears to be the first putative membrane-bound steroid receptor for which at least partial sequence data is available. Falkenstein et al. propose that the progesterone binding protein may function in mediating rapid non-genomic steroid effects that are distinct from the known classic intracellular steroid receptor pathways (described in Fuller, P. (1991) FASEB J. 5:3092-3099).
The close identity of 25-Dx and the progesterone binding protein suggest a possible linkage between cytokine receptor-meditated signal transduction and steroid signaling pathways in the development of both neoplastic and inflammatory responses. For example, a high level of internal homology exists among 25-Dx, the progesterone binding protein, and the transmembrane domain of the IL-6 receptor (Selmin et al., supra and Falkenstein et al., supra). The interaction of the IL-6 receptor with its cytokine is critical in polyclonal B-cell activation, and IL-6 levels are elevated in a number of inflammatory, autoimmune and malignant states including cervical carcinoma, lymphoma and myeloma. Elevated IL-6 levels in transgenic mice result in massive plasmacytosis and mesangial proliferative glomerulonephritis. Interestingly, loss of an estrogen source as occurs in ovariectomy of mice produces an elevation in IL-6 levels and subsequent development of a osteoclastogenic osteoporosis (Narazaki, M. and Kishimoto, T. (1994) In: Nicola, N. (ed.) Guidebook to Cytokines and their Receptors, Oxford University Press, New York, N.Y., pp. 56-61).
Discovery of a new human cytokine/steroid receptor protein satisfies a need in the art by providing new compositions useful in diagnosing and treating disorders associated with aberrant cellular development and differentiation and inflammation.